Such a mixture is pumped into the container, after which the substances present separate in vertical direction in accordance with their difference in specific weight. After a period of time layers distinguishable in vertical direction are situated in the container, wherein it is desirable to be able to determine which substance each layer comprises and what the relevant height of each layer is. A known method for doing this is to measure an electrical property of the content of the container at different heights, using a measuring device for local measurement of at least one electrical property of the content of the container. The measuring device here comprises at least three electrodes disposed adjacently of each other in height direction and electrically insulated from each other, wherein each of the electrodes can be connected to an electrical measuring circuit for measuring, via one of the at least three electrodes and at least one other of the at least three electrodes which is connected to an electrical earth, the electrical property of the content of the container in the vicinity of the electrode connected to the measuring device and the at least one electrode connected to the electrical earth. The electrical measuring circuit is here adapted to generate at least one electric measuring signal representing the measured electrical property, and the device comprises a control unit for connecting at least one of the at least three electrodes to the electrical measuring circuit and connecting at least one other of the at least three electrodes to an electrical earth.
Such devices are known from, among others, the European patent application EP-A-1 744 132 which describes a capacitive level sensor, wherein an electric probe with a number of electrodes is present which, using a measuring circuit, can measure an electrical capacitance relative to a common electrical earth.
The American patent U.S. Pat. No. 6,164,132 describes a capacitive liquid level indicator wherein a number of adjacent electrodes are arranged on a substrate, and wherein each electrode is coupled to an electric signal detection circuit.
The known devices have the drawback however that they do not accurately measure the electrical property, in particular the capacitance, of the content of the container to be measured, since the measurement is subject to influences from outside the immediate vicinity of the electrodes.